We report the first detailed characterization of the sheet third-harmonic optical susceptibility, χ(3)s, of tungsten diselenide (WSe2). With a home-built multiphoton microscope setup developed to study harmonics generation, we map the second and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single- and few-layers flakes of WSe2. We register a value of |χ(3)s| ≈ 0.9 × 10−28 m3 V−2 at a fundamental excitation frequency of ℏω = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded sheet second-harmonic susceptibility is found to be |χ(2)s| ≈ 0.7 × 10−19 m2 V−1 in very good agreement on the order of magnitude with recent reports for WSe2, which asserts the robustness of our values for |χ(3)s|.

Reports on micromachining, [11,12] atomic healing, [13] nanoparticles decoration, [14,15] lateral heterostructures, [16,17] and heterocrystals [18] have demonstrated the feasibility of defects manipulation and postprocessing techniques. All those results have been summarized in important recent reviews on graphene [19] and TMDs. [20,21] One very important aspect is how the defect-engineered and postprocessed materials are characterized. The most widely used characterization techniques are scanning transmission electron microscopy (STEM) [2] for atomically resolved analysis of crystalline structure and different material phases, and PL emission [3,5] for micrometric-resolved linear optical characterization. However, to the best of our knowledge, no report has yet explored the potential of second-harmonic generation (SHG) as a characterization tool for point defects in TMDs. SHG has been extensively applied in studies about nonlinear optical properties of 2D materials, [22][23][24][25][26] mostly related to crystal symmetry investigation, grain boundaries characterization, [27] and in van der Waals heterostructures properties. [28] Particularly for monolayer TMDs, which lack inversion symmetry, changes in SHG signal can be used to track the density of defects in the material as compared to the pristine sample, with spatial resolutions typically at the order of sub-micrometer size for diffraction-limited laser beams.Lateral homojunctions in as-grown chemical transport deposition monolayer tungsten disulfide (WS 2 ) have been recently reported by Liu et al. [29] These homojunctions, as observed by fluorescence microscopy, may give rise to PL concentric patterns of alternating bright and dark regions. Further energydispersive X-ray spectroscopy (EDX) analysis and density functional theory (DFT) calculation revealed that this phenomenon is due to chemical heterogeneity: bright PL areas have less sulfur (S) vacancies, and since it is dominated by excitons, the PL emission is strong; dark PL areas have more S vacancies, increasing the density of mid-gap states, changing the bandgap from direct to indirect, and making PL emission weaker.The modulation in the density of defects should also impact the nonlinear optical properties of monolayer WS 2 crystal, since the second-order susceptibility of the material may experience an improvement in the infrared due to the presence of midgap states related to defects. In this work, we report SHG as Defects engineering in transition metal dichalcogenides is a topic of intense research recently, since crystal properties can be controlled and tailored during and after fabrication. In this context, defects characterization is key to understand the material structure and enable specific applications. In this work, second-harmonic generation (SHG) spectroscopy is used to map concentric triangular defective regions in as-grown monolayer tungsten disulfide, demonstrating that SHG can be used for defects observation and characterization in layered noncentrosymmetric nanomaterials. In monola...

A population imbalance at different valleys of an electronic system lowers its effective rotational symmetry. We introduce a technique to measure such imbalance (a valley polarization), which exploits the unique fingerprints of this symmetry reduction in the polarization-dependent secondharmonic generation (SHG). We present the principle and detection scheme in the context of hexagonal two-dimensional crystals, which include graphene-based systems and the family of transition metal dichalcogenides, and provide a direct experimental demonstration using a molybdenum diselenide monolayer with 2H polytype at room temperature. We deliberately use the simplest possible setup, where a single pulsed laser beam simultaneously controls the valley imbalance and tracks the SHG process. We further developed a model of the transient population dynamics, which analytically describes the valley-induced SHG rotation in very good agreement with the experimental data. In addition to providing the first experimental demonstration of the effect, this work establishes a conceptually simple, compact, and transferable way of measuring instantaneous valley polarization, with direct applicability in the nascent field of valleytronics.

This paper presents, for the first time, the successful transfer of exfoliated monolayer graphene flake to the optical fiber end face and alignment to its core. By fabricating and optimizing a polymeric poly (methyl methacrylate) (PMMA) and polyvinyl alcohol (PVA) substrate, it is possible to obtain a contrast of up to 11% for green light illumination, allowing the identification of monolayer graphene flakes that were transferred to optical fiber samples and aligned to its core. With Raman spectroscopy, it is demonstrated that graphene flake completely covers the optical fiber core, and its quality remains unaltered after the transfer. The generation of mode-locked erbium-doped fiber laser pulses, with a duration of 672 fs, with a single-monolayer graphene flake as a saturable absorber, is demonstrated for the first time. This transfer technique is of general applicability and can be used for other twodimensional (2D) exfoliated materials.

We present a study of pulse generation and propagation in erbium-doped fiber lasers with cavity length varying from 8 m to 3.5 km. We demonstrate that soliton effect determines the pulse stabilization in ultralong cavities, measuring pulses with an average 7.0 ps pulsewidth for cavity lengths between 2.25 and 3.5 km. We also demonstrate that, by filling fundamental soliton requirements, pulsewidth can be determined by length and total dispersion cavity parameters.